1. Field of the Invention
The present invention relates to image-heating devices for heating images formed on recording media in image-forming apparatuses using electrophotography or the like.
More particularly, the present invention relates to an image-heating device having a plurality of image-heating units in tandem for heating an image formed on a recording medium, wherein the recording medium is caused to pass through an image-heating unit positioned upstream in a conveying direction of the recording medium and then through an image-heating unit positioned downstream in the conveying direction of the recording medium.
2. Description of the Related Art
In known electrophotographic technology, an electrostatic latent image formed on a photosensitive drum that functions as an image carrier is developed with toner, and then the thus obtained toner image is transferred onto a recording medium, such as a piece of paper or an overhead projector (OHP) sheet, directly or using an intermediate transferring member. Then, the image is fixed on the recording medium by applying heat and pressure with a fixing unit.
Recently, electrophotographic image-forming apparatuses have come into common use in place of printing apparatuses. Accordingly, image-forming apparatuses including a plurality of image-heating units to output an image with a large glossiness range at a high speed have been suggested in, for example, Japanese Patent Laid-Open No. 2002-214948.
In known apparatuses including only one fixing unit, the glossiness range of the output image is small since there is a limit to the amount of heat that can be applied to the recording medium to melt the toner. In addition, since the amount of heat required differs depending on the type of recording medium, highly glossy images cannot be formed on a recording medium that requires a large amount of heat. In comparison, when a first fixing unit and a second fixing unit are provided at upstream and downstream positions, respectively, and settings of image-heating units included in the first and second fixing units are variable, the range of heat that can be applied to the recording medium is increased. Accordingly, the glossiness range of the output image can be increased.
On the other hand, Japanese Patent Laid-Open Nos. 8-227194, 9-190111, and 2002-091211 disclose structures including a single image-heating unit. In these structures, the glossiness of an image output from the image-heating unit is detected, and then the settings of the image-heating unit are adjusted such that the detected glossiness approaches a desired glossiness. Accordingly, the quality of the output image is improved.
Similarly, in the above-described structures including a plurality of image-heating units, the quality of the output image can be improved by detecting the glossiness of the image that is finally output after being heated by the plurality of image-heating units and adjusting the settings of one of the image-heating units.
However, when the glossiness of the image is detected after the image passes through all of the image-heating units and is finally output, a relatively long time is required to adjust the settings of the image-heating units since it takes a longer time for the image to be finally output after passing through all of the image-heating units, as compared to an image-forming apparatus with only a single image-heating unit. Accordingly, there is a demand to perform the adjustment at an intermediate position before the image is finally output.